I assume that the gate in question is going to be outside exposed to the elements. If that's the case a miter is very likely going to fail quickly. You pretty much have to take into account the weight of the gate and how often the gate is going to be opened and closed. Adding screws is also going to give water new places to seep into the wood unless you find a way to seal off those cracks and crevices..
Thinking about gates..I really need to replace all of them on the property here. 3 double panel gates and 1 single..for that matter the entire fence needs replaced..It's all rotting away bit by bit. I wish I had never opened this thread. Now I have more to worry about this year and next..lol
I figured it's time to change my signature so hold your breath. This is it.
Marty or Marty Farty if you feel mean.